Piezoelectricity refers to a phenomenon observed in some materials in which imposition of a stress will establish an electric field whose intensity is proportional to the stress level. Jacques and Pierre Curie discovered piezoelectricity in quartz in 1880; but the materials most often used today as piezoelectrics are barium titanate (BaTiO3) and lead zirconate titanate (PZT). Both are ceramic materials which require high temperature processing in the presence of a high electric field to render them piezoelectric. They tend to be expensive and brittle materials. (See Rosen, C. Z., B. V. Hiremath, and R. E. Newnham, eds. Piezoelectricity. 1992, American Institute of Physics: New York for a review of piezoelectricity.)
Piezoelectric materials exhibit a linear coupling between a stress field and an electric field. Piezoelectricity occurs when a mechanical field induces an electrical field, or vice versa. Generally, transduction from a mechanical signal to an electrical signal is referred to as sensing, whereas transduction using electrical input to produce a mechanical output is referred to as actuation. In order to fully investigate piezoelectric materials, both sensing and actuation should be considered, because some measurements are easier to make on sensors and others on actuators.
BaTiO3 and PZT were first discovered in the late 1940s and early 1950s (Jaffe, B. 1955: U.S. Pat. No. 2,708,244, issued May 10, 1955; Gray, R. B. 1949: U.S. Pat. No. 2,486,560, issued Nov. 1, 1949). Efforts since then to find new piezoelectric materials generally have met with disappointment. The most promising development was the discovery of piezoelectricity in PVDF, but this polymer loses its piezoelectricity at a relatively low temperature (70° C.) and requires uniaxial or biaxial stretching in order to introduce piezoelectricity (Kawai, H., The Piezoelectricity of Poly(vinylidene Fluoride). Jpn. J. Appl. Phys., 1969. 8: p. 975). Mechanical fatigue is also a problem with PVDF. Few commercial products using piezoelectric PVDF have been marketed, although the military has employed thick PVDF hydrophones.
Piezoelectricity can be observed in polypropylene foam, often referred to as LDPP for low density polypropylene. (See Gerhard-Multhaupt, R. Voided polymer electrets—New materials, new challenges, new chances. in 11th International Symposium on Electrets. 2002 for a review.) LDPP is produced in a blow-extrusion process that results in polypropylene with closed cell spherical voids. The material is then biaxially stretched to produce disk-shaped voids. It is exposed to corona charging at levels of about 20 kV that cleaves the molecular bonds of the gas trapped in the voids yielding a d33 of up to 300 pC/N. LDPP has some inherent problems that limit its utility. First, it loses its piezoelectric function starting at about 50° C. This means that the material is inappropriate for any use that will cause significant warming (potentially any operation in air, for instance). Second, at high pressures, it is likely that the relatively low stiffness of the air voids compared to the polymer will result in collapse of the voids, possibly with discharging. Thus it is not appropriate for high pressure use.
Composites formed by placing a piezoelectric material in a polymer matrix have also been pursued successfully for many years. The bulk of the work has been on 1-3 composites, in which rods of piezoelectric materials (PZT or BaTiO3) are embedded in a polymer matrix. Applications of piezoelectric 1-3 composites have focused on sonar although there has been increasing interest in their use for nondestructive evaluation of structures and acoustic monitoring of faults in the nuclear industry (Fleury, G. and C. Gondard, Improvements of Ultrasonic Inspections through the Use of Piezo-Composite Transducers. Transducer Workshop, 1996). Compared to the standard piezoelectric materials, 1-3 composites are lower in mass and more rugged. Volume fractions of the ceramic component vary from 0-50% with thicknesses ranging from fractions of a millimeter to 25 millimeters (Benjamin, K., Recent Advances in 1-3 Piezoelectric Polymer Composite Transducer Technology for AUV/UUV Acoustic Imaging Applications. J. Electroceramics, 2002. 8: p. 145). The material typically is produced using an injection molding process to produce ceramic rods in a pattern with a plate structure at one end to keep the rod spacing and alignment fixed. A polymer then fills the regions between the rods, and the plate end is sliced off.
Piezoelectric materials are the key components of electromechanical transducers (sensors and actuators) for automatic control systems, and measurement and monitoring systems. Electromechanical transducers can be found in items ranging from hearing aids to automobiles, from clothing dryers to perimeter sensors, and from elevators to computers. The history of transduction reads like a timeline for materials invention with each new coupling mechanism discovery leading to new devices (Busch-Vishniac, I. J., Electromechanical Sensors and Actuators. 1999, New York: Springer).
The most common acoustic transducers are microphones and loudspeakers. They are found in every telephone, in tape and digital audio recorders, and increasingly in automobiles, where they are being used for hands-free communication and in monitoring engine performance. Today, most common microphones are electret microphones. Electret materials are those which exhibit a permanent polarization or space charge. First reported in 1962 (Sessler, G. M. and J. E. West, Self-Based Conderser Microphone with High Capacitance. J. Acoust. Soc. Am., 1962. 34: p. 1787), electret microphones use a membrane suspended under tension above a rigid backplate, a perforated backplate and back cavity to reduce stiffness, and a small hole through the structure for dc pressure equalization.
By contrast, a piezoelectric microphone can be much simpler in structure. The piezoelectric material serves as the dielectric element, with a metal surface on top and bottom. It is unnecessary to supply any tension, to vent the device, or to provide a back cavity and perforated backplate. The result is a very simple microphone in which the material is contained either in a ring allowing sound access from both sides (a gradient microphone) or in a cylinder closed at one end (conventional pressure microphone). While it is possible to make piezoelectric microphones from BaTiO3 and PZT, they are generally less sensitive and more expensive than electret microphones.